gaara! did you watch the flashback of his childhood?? super depressing. everyone was afraid of him, even his own family. he was rejected over and over.
naruto at least still had iruka sensei, the sandaime.. i think he was also friends (maybe not very close, but still friendly) with shikmaru, chouji and kiba because they were like the last few in class weren't they?
sasuke is lonely but he closed himself to the world, he wouldn't give a chance to anybody who tried to come close so his loneliness was his own choice U_U